The present invention relates generally to digital communications. More specifically, the present invention relates to a system in which variable rate data is transmitted without an indication of the data rate and is received at a communications receiver where the data rate of the transmitted data is determined for use in processing the data.
In digital communications systems, particularly those which use spread spectrum modulation, a transmitter may employ a vocoding system which encodes voice information at a variable rate to lower the data rate during pauses or other absences of voice activity, thereby reducing the level of interference caused by this transmitter to receivers other than the intended receiver. At the receiver, or otherwise associated with the receiver, a vocoding system is employed for reconstructing the voice information. It should be understood that in addition to voice information, non-voice information alone or a mixture of the two may be transmitted to the receiver.
A vocoder which is suited for application in this environment is described in U.S. Pat. No. 5,414,796, issued May 9, 1995, entitled "VARIABLE RATE VOCODER," and assigned to the assignee of the present invention. This vocoder produces from digital samples of the voice information encoded data at four different rates, e.g. approximately 8,000 bits per second (bps), 4,000 bps, 2,000 bps and 1,000 bps, based on voice activity during a 20 millisecond (ms) frame. Each frame of vocoder data is formatted with overhead bits as 9,600 bps, 4,800 bps, 2,400 bps, and 1,200 bps data frames. The highest rate data frame which corresponds to a 9,600 bps frame is referred to as a "full rate" frame; a 4,800 bps data frame is referred to as a "half rate" frame; a 2,400 bps data frame is referred to as a "quarter rate" frame; and a 1,200 bps data frame is referred to as an "eighth rate" frame. In neither the encoding process nor the frame formatting process is rate information included in the data.
Additional details on the formatting of the vocoder data into data frames are described in copending U.S. patent application Ser. No. 08/171,146, filed Dec. 21, 1993 which is a continuation of U.S. patent application Ser. No. 07/822,164, entitled "METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION," filed Jan. 16, 1992, now abandoned, and assigned to the assignee of the present invention. The data frames may be further processed, spread spectrum modulated and transmitted as described in U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM," issued Apr. 7, 1992, and assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein.
Since rate information for each frame is not transmitted, the receiver must determine from the received frame of data the rate at which they were encoded in order for the vocoder to properly reconstruct the voice information. Although the transmitter could transmit information regarding the rate at which the frame was encoded this would reduce system resources available for transmitting voice and non-voice data. Furthermore corruption in the transmitted rate information would adversely affect the entire frame. Thus, it is desirable for the receiver to determine the rate at which the frame was encoded without receiving rate information from the transmitter. These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.